The discussion revolves around the relationship between vacuum energy and dark energy, exploring whether vacuum energy could serve as an explanation for dark energy's effects in the universe. Participants delve into theoretical implications, calculations, and the nature of dark energy in relation to gravitational energy, with a focus on cosmological concepts and the expansion of the universe.
Participants do not reach a consensus, with multiple competing views on the relationship between vacuum energy and dark energy, as well as the nature of dark energy itself. Disagreements persist regarding the implications of gravitational effects and the interpretation of relativistic velocities.
Limitations include unresolved mathematical calculations regarding vacuum energy, assumptions about the nature of dark energy, and the complexities of relativistic effects on galaxies. The discussion also highlights the dependence on definitions of energy and gravitational interactions.
The numbers don't work out, yes. But vacuum energy is some 120 orders of magnitude too much to account for dark energy.zhermes said:Short answer: the numbers don't work out. Vaccuum energy isn't enough to account for the effects of dark energy.
John232 said:Does dark energy act the same as gravitational energy? Accoring to Berkley, the universe can expand faster than the speed of light. Then the edge of the visable universe has galaxies that travel almost the speed of light. So then what if the universe was larger than the visable universe? Then wouldn't galaxies be traveling faster than the speed of light with infinite relative mass?
This is completely wrong, and even more so misleading.physixlover said:1.Gravity is an effect caused by the accelerating movement of dark energy.
and 2.yes, offcourse the galaxies will be traveling at speed of light.
NO! Dark Energy and gravitational energy (lets say, normal mass energy---along with its gravitational effects) are just about opposites, in numerous ways. The most important is that dark energy is repulsive while gravity is universally attractive. The other differences aren't especially relevant to this discussion, but they are important.John232 said:Does dark energy act the same as gravitational energy?
The universe is almost certainly much, much larger than the visible universe.John232 said:So then what if the universe was larger than the visable universe? Then wouldn't galaxies be traveling faster than the speed of light with infinite relative mass?
We should be a little careful here. Dark energy isn't repulsive anymore than ordinary matter is attractive. Rather, a homogeneous distribution of dark energy will cause spacetime to accelerate -- this is a gravitational phenomenon. So it's more correct to say that dark energy is gravitationally repulsive, and ordinary matter is gravitationally attractive.zhermes said:NO! Dark Energy and gravitational energy (lets say, normal mass energy---along with its gravitational effects) are just about opposites, in numerous ways. The most important is that dark energy is repulsive while gravity is universally attractive. The other differences aren't especially relevant to this discussion, but they are important.
To add to zhermes' good answer, it's important to point out that there is a distance from Earth at which objects are receding at light speed -- this is the Hubble radius. The Hubble radius exists regardless of whether there is any dark energy present (or in other words, whether inflation is happening). And yes, beyond this point, galaxies are receding superluminally. As zhermes points out, two distant galaxies do not share a local frame, and so special relativity is not applicable. In fact, each galaxy is locally at rest -- instead one can view the space between the galaxies as expanding. So distant galaxies recede faster the farther away they are from us simply because there is more space expanding between us and them.John232 said:So then what if the universe was larger than the visable universe? Then wouldn't galaxies be traveling faster than the speed of light with infinite relative mass?
The addition of velocities only makes sense locally. Technically, the two objects whose velocities you wish to add must exist within the same tangent space. This is in general not true for distant objects in the universe because of the existence of spacetime curvature. Your reasoning makes sense if you apply special relativity to the cosmos -- however, this is not the correct way to formulate cosmology because special relativity is not compatible with gravity. For cosmology, we need to use general relativity.John232 said:I try to avoid talking about objects traveling faster than light on the opposite side of the universe to avoid any addition of velocity issues. The fact of the matter is that if we where to assume that we are at the center of the universe, then a galaxy on the edge of the visable universe would be traveling close to the speed of light already relative to us.
Why do you think that by adding mass to astrophysical objects you can match the cosmological observations that are currently accommodated by dark energy and dark matter? Why do you think dark energy and dark matter are conceptually related? Aside from the word 'dark', they need have nothing to do with each other.The main question I have been haveing lately is if galaxies traveling along with the expansion of space are null and void of any relativistic effects. It seems to me that a lot of the problems scientist have with dark energy/matter could be explained just by adding in relativistic mass to these systems. There just isn't the information put out there about it to be able know about how relativity affects galaxies traveling at relativistic speeds.
It's all about balance. In a closed universe with the correct amount of matter, radiation, and cosmological constant, you can achieve a static universe. Today, we know that the observable universe is very close to flat, and dark energy dominates the energy density. Put that kind of matter/energy content into Einstein's Equations and you get an accelerating universe.What was so cosmo- -logical about dark energy? I read once that the cosmological constant was an accurate depiction of dark energy. What did Einstein know about the universe that lead him to predict the presence of a force overwhelming the universe that everyone at the time was completely unaware of? Everyone says that he was only trying to balance the force of gravity so that it created a static universe, but the universe was larger than he predicted and I read that the cosmological constant actually described an expanding universe if applied to it today.
I like your idea, but this was all taken into account when the cosmologists did these studies. It's not as simple as just adding relativistic mass to things, since, as I mentioned above, special relativity is not relevant to non-inertial systems. However, when general relativity is used to understand galaxy rotation (which I believe is what you are referring to), one finds that indeed the angular speed of rotation does not match that expected from the matter density at the center of the galaxy. Dark matter was initially introduced to deal with this problem; however, it's important to note that it has since come to be necessary in many other parts of cosmology, notably, in the formation of galaxies and galaxy clusters. People have also tried modifying the gravity theory to understand the rotation problem, but recent evidence for particulate dark matter (see Bullet Cluster) and other theoretical shortcomings make particulate dark matter, in my opinion, the more likely explanation.Then it occurred to me one day while I was watching a TV episode about how cosmologist measure the mass of systems by comparing its orbit to other nearby systems. What if its relative velocity made it appear more massive to other systems traveling at relative speeds and didn't have a relativistic mass to the other bodies that it was in orbit with that was not at relativistic speeds. If that was the case then it would seem that according to relativity that we should have dark matter and dark energy that I never could believe that should be existing...
This is simply wrong. Gravitational forces and fields are always considered to have the same sign (negative) whatever way they are produced:by "normal" matter or by any type of energy density (i.e. EM radiation etc). Perhaps your confusion comes from the fact that fermionic matter stress-energy tensor has positive pressure in its trace and the vacuum/dark energy tensor has negative pressure. But the truth is there is no such thing as a gravitational "repulsive" force, any repulsive force couldn't be gravitational in nature. When dark energy is said to accelerate spacetime is just a form of stating the equivalence principle by which a gravitational field is indistinguishible from a uniformly accelerated frame.zhermes said:NO! Dark Energy and gravitational energy (lets say, normal mass energy---along with its gravitational effects) are just about opposites, in numerous ways. The most important is that dark energy is repulsive while gravity is universally attractive.
mr.dark said:well, what if dark energy is, in fact, the combination of smaller, simpler things and the reason they don't have an answer for it is simply because they haven't thought of it that way?? they continue looking for one, large thing to be the cause of it, so i highly doubt they have looked at smaller components adding up to dark energy.
TrickyDicky said:This is simply wrong. Gravitational forces and fields are always considered to have the same sign (negative) whatever way they are produced:by "normal" matter or by any type of energy density (i.e. EM radiation etc). Perhaps your confusion comes from the fact that fermionic matter stress-energy tensor has positive pressure in its trace and the vacuum/dark energy tensor has negative pressure. But the truth is there is no such thing as a gravitational "repulsive" force, any repulsive force couldn't be gravitational in nature. When dark energy is said to accelerate spacetime is just a form of stating the equivalence principle by which a gravitational field is indistinguishible from a uniformly accelerated frame.
The cosmological constant is simply a constant in Einstein's Equations, and can have any value whatever. I believe what you are referring to is the vacuum energy as calculated from the Standard Model of particle physics, which gives a CC 120 orders of magnitude too large. This is mitigated somewhat when supersymmetry is taken into account. Correct, this is a problem, but my statement was simply that the CC behaves like the energy of the vacuum -- which vacuum is unclear at this point. Quintessence, in which the dark energy is due to the vacuum energy of a rolling scalar field, could still explain dark energy. Granted, the potential would need to be fine tuned (although the degree of tuning and whether or not the potential is stable to radiative corrections is a model dependent factor) but a quintessence field with a simple potential is still 'simple' in my opinion. I still don't know what about dark energy you think makes it such a 'big thing'.mr.dark said:in my opinion, it can be many things. true, the cosmological constant is the simplest explanation; however, as stated in many reports, the cosmological constant is still a ways of by a power of about twenty one of ten i believe (or somewhere around there), meaning that instead of coming up short as some theories do, it overshoots, like the vacuum energy.
Why do you think reducing mass will explain the current accelerated expansion of the universe?what i was thinking, however, was if they can find something that can take away mass or if they added dark matter to vacuum energy and found some type of molecule or atom or such that is able to reduce the total mass of an energy or form of matter, they may find the answer.
Because the stress-energy associated with black hole spin does not lead to accelerated expansion.Imax said:What if dark energy = black hole spin?
This is not true. Dark energy is just as gravitational as ordinary matter. It is gravitationally repulsive whereas ordinary matter is attractive.mr.dark said:this is true. dark energy is able to travel and dispel gravity, allowing it to continue to travel through objects. anything that has the opposite affect of gravity's attractive nature is not considered gravitational.
Thanks for the reference Chronos. This is a much more rigorous way to talk to about model simplicity.Chronos said:I encourage everyone interested to read this paper - http://arxiv.org/abs/astro-ph/0703191
Correct.bapowell said:Dark energy is just as gravitational as ordinary matter.
Are you proposing a modified gravity that is repulsive instead of atractive? I'd say that would be ATM as well as wrong.bapowell said:It is gravitationally repulsive whereas ordinary matter is attractive.
No idea what this has to do with Automatic Teller Machines. I am not proposing a modified gravity -- just GR. In the weak field limit in the presence of a CC, \Lambda, one has for the Newtonian potential \Phi:TrickyDicky said:Are you proposing a modified gravity that is repulsive instead of atractive? I'd say that would be ATM as well as wrong.
bapowell said:No idea what this has to do with Automatic Teller Machines. I am not proposing a modified gravity -- just GR. In the weak field limit in the presence of a CC, \Lambda, one has for the Newtonian potential \Phi:
\nabla^2 \Phi = -\nabla g = 4\pi G \rho - \Lambda
where g is the gravitational acceleration. This becomes
g = -\frac{GM}{r^2} + \frac{\Lambda r}{3}
and you can see the \Lambda is a repulsive contribution.
Now I'm confused. Dark energy is vacuum energy. The cosmological constant that I am considering (\Lambda above) is of course only one special example of dark energy -- that of constant density. However, whether dark energy is dynamical or constant, the effective equation of state is still such that \rho \sim -p, and my above example is valid (give \Lambda a time dependence -- you'll reach the same conclusion as the constant case.) Whatever the form of dark energy, the gravitational forces are repulsive as I have just demonstrated. It should be well known to you that vacuum energy leads to an accelerated expansion of spacetime. I am not assuming that Einstein Equations necessarily contain \Lambda, merely demonstrating that if they do, then there is a repulsive contribution.TrickyDicky said:Actually I was assuming the thread's hypothesis that dark energy was actually the vacuum energy, in that case the the gravitational force would be atractive as it happens with any other energy.
You are making different assumptions as I can see, first that dark energy is Lambda, second that GR field equations must contain Lambda. Both are valid assumptions but not necesarily correct, especially if the first is not right,decreases the probabilities of the second to be right. Besides, Lambda repulsive contribution might not be gravitational, but some yet unknown interaction type if we must speculate further about this speculative "dark energy" thing.
The hypothetical assumption I used surely needs not be correct either.
I tend to agree, but it is currently just an educated guess. Dark energy is still puzzling for mainstream science.bapowell said:Dark energy is vacuum energy.
bapowell said:I have no idea what you mean by the repulsive nature of \Lambda might not be gravitational.
OK, and I'm saying it's not an educated guess as to how vacuum energy behaves gravitationally. I've proven this with my above example. Please show explicitly how vacuum energy density is gravitationally attractive.TrickyDicky said:I tend to agree, but it is currently just an educated guess. Dark energy is still puzzling for mainstream science.
But in any case vacuum energy can only produce an atractive gravitational field just like any other energy (EM radiation, etc) until proved otherwise.
So you're saying flat and negatively curved spaces are incompatible with GR? I hope not. I don't see how adding a CC to Einstein's Equations and working out the dynamics of such a universe is wild speculation.I'm just saying that if we abandon wild speculations for a moment, to this day gravitational force is atractive, and is defined by being atractive, accordin to GR matter-energy Einstein tensor curves spacetime only with positive curvature
OK, but Lambda -- whether you want to call it dark energy or not -- Lambda, a constant in Einstein's Equations, gives accelerated expansion if it dominates the energy density. It's not a matter of debate or speculation. I agree that dark energy might not be gravitational, but that's not what you said. You argued that Lambda -- the constant in Einstein's Equations -- and its repulsive gravitational contribution was not gravitational. I'm still confused by that statement.(let's stick to the original GR equations without Lambda until it is completely accepted that Lambda=acelerated expansion of space) nobody can say with certainty that "dark energy" is gravitational since nobody knows as of today what "dark energy" is.
Maybe the problem is semantic, when you say that vacuum energy is repulsive, do you mean that since its presure is negative it produces negatively curved space? I could agree with that.bapowell said:Please show explicitly how vacuum energy density is gravitationally attractive.
I hope not, too, I meant "normal" matter stress-energy curves positively space. Sorry I slipped that.bapowell said:So you're saying flat and negatively curved spaces are incompatible with GR? I hope not.
I was identifying dark energy" unknown nature with Lambda, and admitting we don't know yet much about it, I didn't mean to confuse, so never mind that statement.bapowell said:You argued that Lambda -- the constant in Einstein's Equations -- and its repulsive gravitational contribution was not gravitational. I'm still confused by that statement.
No. Vacuum energy has negative pressure. Dust and radiation have zero and positive pressure, respectively. From Einstein's equations in the Newtonian limit, I have shown in my previous post that the vacuum energy (represented by Lambda) contributes oppositely to the gravitational potential from ordinary matter/radiation. This is what I mean by repulsive -- the presence of vacuum energy lessens the acceleration due to gravity felt by two test masses. Taken by itself, it gives rise to a springy repulsion.TrickyDicky said:Maybe the problem is semantic, when you say that vacuum energy is repulsive, do you mean that since its presure is negative it produces negatively curved space? I could agree with that.